1. Field of the Invention
The present invention relates to a flexible tube and, more specifically, to a flexible tube made of synthetic resins and having outstanding resistance to pressure and high flexibility.
2. Description of the Related Art
As shown in FIG. 6, one conventional flexible tube is composed of a tube wall 33 formed by surrounding a spiral reinforcement 31 made of a hard material with a soft material 32. Such a conventional flexible tube 41 has high flexibility because the reinforcement 31 has a helical shape and the tube wall 33 includes the soft resin portion 32. However, when a pressure of the fluid in the tube increases, the reinforcement 31 is subjected to a strong force which acts to untwist the helix thereof. In such cases, since the tube wall 33 does not have sufficient rigidity to resist the force and since the tube wall 33 including the spiral reinforcement 31 is subjected to large twist and elongation, high internal pressure can not be applied thereto.
Accordingly, it is an object of the present invention to provide a flexible tube having sufficient flexibility and high resistance to internal pressure, wherein the amount of twist and elongation due to internal pressure is small.
The present invention provides to such a flexible tube as shown in FIG. 1A for example, which comprises:
an inner tubular layer 11 made of a soft resin;
an inner reinforcement 1 made of a hard resin, said inner reinforcement being helically wound around the inner tubular layer 11;
an intermediate tubular layer 12 made of a soft resin, said intermediate tubular layer surrounding the inner reinforcement 1; and
an outer reinforcement 4 made a hard resin, said outer reinforcement being helically wound around the intermediate tubular layer 12 in the direction opposite to the winding direction of the inner reinforcement 1.
As a consequence of the above construction, the flexible tube has improved pressure resistance while keeping flexibility. Further, the amount of twist and elongation due to pressure applied by fluid in the flexible tube is small, because the flexible tube includes an inner tubular layer made of a soft resin, an inner spiral reinforcement made of a hard resin, an intermediate tubular layer made of a soft resin and an outer spiral reinforcement made of a hard resin, and because the winding direction of the inner reinforcement and that of the outer reinforcement are opposite to each other.
The flexible tube may be provided with an outer tubular layer 13 made of a soft resin to surround the outer spiral reinforcement. Preferably, the inner tubular layer and the intermediate tubular layer are bonded to each other with the soft resin filling the gap between the spirals of the inner spiral reinforcement.
In another aspect, the present invention provides a flexible tube in which each of the inner and outer reinforcement is in the form of a flat strip. Because of the large proportion of the inner and outer spiral reinforcement relative to the soft resin portion, the flexible tube has high resistance to pressure and the amount of elongation and twist thereof is small.
In still another aspect, the present invention provides such a flexible tube as shown in FIG. 2 for example, which comprises:
an inner tube 14 formed of a first tape 3, the first 3 tape having an inner hard resin reinforcement 1 covered with a soft resin 2, being helically wound, and side end portions of the wound first tape 3 being bonded to each other; and
an outer tube 15 formed of a second tape 6, the second tape 6 having an outer hard resin reinforcement 4 covered with a soft resin 5, being helically wound around the inner tube 14 in a direction opposite to the winding direction of the first tape 3, and side end portions of the wound second tape 6 being bonded to each other;
wherein an outer surface 14A of the inner tube 14 and an inner surface 15B of the outer tube 15 are bonded to each other to integrate the inner and outer tubes 14 and 15 into a unitary body as a whole.
Since the flexible tube has a double reinforcement structure which extends through the tube wall of a soft resin and in which inner and outer reinforcements made of a hard resin are helically wound in the opposite direction to each other, there can be accomplished high resistance to pressure, reduced twist and elongation when applied with an internal pressure and excellent bending properties.
In the above flexible tube, the outer reinforcement may be exposed around the periphery. Because the outer reinforcement is exposed around the periphery, the flexibility of the flexible tube is more improved. Also, the flexible tube may be dragged, for example, on the ground with improved slippage.
In a further aspect, the present invention provides a method of manufacturing a flexible tube which comprises:
a first step of covering an inner reinforcement made of a hard resin with a soft resin to form a first tape having side end portions;
a second step of helically winding the first tape and bonding the side end portions of the wound first tape to each other to form an inner tube;
a third step of covering an outer reinforcement made of a hard resin with a soft resin to form a second tape having side end portions;
a fourth step of helically winding the second tape around the inner tube in the direction opposite to the winding direction of the first tape and bonding the side end portions of the wound second tape to each other to form an outer tube; and
a fifth step of bonding an outer surface of the inner tube and an inner surface of the outer tube to each other to integrate the inner and outer tubes into a unitary body.
In the above method, a flexible tube having a double reinforcement structure in which inner and outer reinforcements are helically wound in the opposite direction to each other can be manufactured with a high efficiency.
The expression xe2x80x9ccovering an inner or outer reinforcement with a soft resinxe2x80x9d as used herein includes the case where the soft resin covers a part of the reinforcement with the rest portion being exposed on the surface as well as the case where the reinforcement is surrounded by or encapsulated in the soft resin.